Doximity announces that it has reached the 250,000 member milestone on its physician-only social network. The number accounts for 40 percent of all US physicians and makes Doximity one of the leading online resources used by physicians today. Described as LinkedIn for Doctors, Doximity offers an increasingly persuasive platform for physicians.

Recently, new signups and overall physician usage on the site have hit a tipping point. Analysts at Doximity have been closely tracking physician adoption and comparing it with actual physician usage over time so that they could not only strive to build a large physician base, but also a highly active physician base. In their analysis, they found that as more doctors were signed up to use Doximity at a particular hospital, or in a particular city, the more time each individually doctor used the platform overall. As Doximity’s presence grew in each sub-market, the usage rates for all users increased in parallel.

An example of this is Doximity’s market penetration in Alaska, one of its most active states. With only 1,500 physicians in Alaska, it did not take long for Doximity to reach a critical mass of users and now not only does the company have a high percentage of Alaskan doctors on their system, but they also have a very active community of users. Because of remote locations, physicians often need a way of sending secure email and images to each other to coordinate care across great distances. Doximity became the answer to that problem, as CEO Jeff Tangney explains, “I’d say the main reason we’ve grown so fast is secure email. I can’t imagine doing my job without email, but in fact that’s what we ask every U.S. doctor to do.”

Doximity approached the job of building a user base by attacking sub-markets to generate pockets of highly active users. First win the hospital, then the city, then the state, and eventually the nation. Doximity hits a critical mass within a hospital fast because it offers tangible tools to improve work efficiencies at the local level. Secure email, secure photo sharing, and secure texting, which it says 25 percent of its members use regularly. These tools are all going to be used most frequently by colleagues working on the same patients within the same small sub-market.

On top of secure communications, Doximity brings a flavor of social networking to the physician community, as well as a handful of other useful tools, like: research alerts which work similarly to Google Alerts, where users are alerted when certain types of scholarly articles or studies are published, and a CME credit issuing feature that rewards reading those studies. These tools have helped Doximity grow from having pockets of active users in sporadically placed sub-markets, to having a broad national presence.

The platform seems to have found the right combination of features to attract physicians. According to a 2013 study done by the American College of Physicians, Doximity is now one of the top five most used medical apps for doctors.